Many electrical power sources exist that have high internal resistances. The internal resistance of a power source measures the resistance within that power source to the flow of current through the power source. Internal resistance can be dependent upon many factors, including the construction of the power source, ambient temperature conditions, and changes in the internal chemistry of the power source. Although internal resistance is often associated with a power source comprising batteries, other types of power sources can have relatively high internal resistance. Examples of such power sources include solar cells and fuel cells.
When a power source has a relatively high internal resistance, it is difficult to extract electrical energy from the source in an efficient manner because the power source's internal resistance dissipates a relatively large portion of the electrical energy. That dissipated energy is therefore consumed within the power source and is never delivered to the load. Additionally, it is difficult to use a power source with high internal resistance to provide a desired voltage to a given electrical load, such as a particular electrical or electronic circuit, because the voltage supplied by the power source drops substantially as the load draws current from the power source. If a number of high internal resistance power sources are connected in series (for example, to generate a high output voltage), there are additional losses in terms of extracted power because the current flowing through each power source must pass through the internal resistances of the other power sources that connect to it.
In a circuit arranged to extract energy from a high impedance power source, a number of magnetic components may be utilized to facilitate energy extraction. To ensure efficient operation, it is important that those magnetic components efficiently couple primary and secondary windings to minimize losses within the extraction circuit.